This invention relates to metallic orthopedic implants with load bearing surfaces coated with a thin, dense, low friction, highly wear-resistant, uniformly thick coating of zirconium oxide.
The invention also relates to uniformly thick zirconium oxide coatings on the non-load bearing surfaces of an orthopedic implant where the zirconium oxide provides a barrier between the metallic prosthesis and body tissue thereby preventing the release of metal ions and corrosion of the implant.
The invention also relates to a method of producing a uniformly thick oxide coating on zirconium or a zirconium alloy by controlling the surface roughness of the zirconium or zirconium alloy having a refined microstructure prior to formation of the oxide coating.
The excellent corrosion resistance of zirconium has been known for many years. Zirconium displays excellent corrosion resistance in many aqueous and non-aqueous media and for this reason has seen an increased use in the chemical process industry and in medical applications. A limitation to the wider application of zirconium in these areas is its relatively low resistance to abrasion and its tendency to gall. This relatively low resistance to abrasion and the tendency to gall is also demonstrated in zirconium alloys.
Orthopedic implant materials must combine high strength, corrosion resistance and tissue compatibility. The longevity of the implant is of prime importance especially if the recipient of the implant is relatively young because it is desirable that the implant function for the complete lifetime of a patient. Because certain metal alloys have the required mechanical strength and biocompatibility, they are ideal candidates for the fabrication of prostheses. These alloys include 316L stainless steel, chrome-cobalt-molybdenum alloys and, more recently, titanium alloys which have proven to be the most suitable materials for the fabrication of load-bearing prostheses.
One of the variables affecting the longevity of load-bearing implants, such as hip joint implants, is the rate of wear of the articulating surfaces and long-term effects of the metal ion release. A typical hip joint prosthesis includes a stem, a femoral head and an acetabular cup against which the femoral head articulates. Wear of either or both of the articulating surfaces results in an increasing level of wear particulates and xe2x80x9cplayxe2x80x9d between the femoral head and the cup against which it articulates. Wear debris can contribute to adverse tissue rejection leading to bone resorption, and ultimately the joint must be replaced.
The rate of wear is dependent upon a number of factors which include the relative hardness and surface finish of the material which constitute the femoral head and the acetabular cup, the frictional coefficient between the materials of the cup and head, the load applied and the stresses generated at the articulating surface. The most commom material combinations currently used in fabrication of hip joints implants include femoral heads of cobalt or titanium alloys articulating against acetabular cups lined with organic polymers or composites of such polymers including, e.g., ultra high molecular weight polyethylene (UHMWPE), and femoral heads of polished alumina in combination with acetabular cups lined with an organic polymer or composite or cups made of polished alumina.
Of the factors that influence the rate of wear of conventional hip-joint implants, the most significant are patient weight and activity level. Additionally, heat which is generated by friction in the normal use of the implant as, for instance, in walking has been shown to cause accelerated creep and wear of the polyethylene cup. Furthermore, there is a correlation between the frictional moment which transfers torque loading to the cup and the frictional coefficient between the femoral head and the surface of the acetabular cup against which the head articulates. Cup torque has been associated with cup loosening. Thus, in general, the higher the coefficient of friction for a given load, the higher the level of torque generated. Ceramic bearing surfaces have been shown to produce significantly lower levels of frictional torque.
It is also noteworthy that two of the three commonly used hip-joint systems as indicated above include a metallic femoral head articulating against a UHMWPE liner inside the acetabular cup. UHMWPE, being a polymeric material, is more susceptible to creep when heated than the commonly used metal alloys or ceramics and is consequently more susceptible to wear than the alloys or ceramics.
It has also been found that metal prostheses are not completely inert in the body. Body fluids act upon the metals causing them to slowly corrode by an ionizing process that thereby releases metal ions into the body. Metal ion release from the prosthesis is also related to the rate of wear of load bearing surfaces because the passive oxide film, which is formed on the surface, is constantly removed. The repassivation process constantly releases metal ions during the ionizing process. Furthermore, the presence of third-body wear (cement or bone debris) accelerates this process and microfretted metal particles increase friction. Consequently, the UHMWPE liner inside the acetabular cup, against which the femoral head articulates, is subjected to accelerated levels of creep, wear and torque.
U.S. Pat. No. 415,764 to Suzuki, et al. recognizes that while metal prostheses have excellent mechanical strength they tend to corrode in the body by ionization. Suzuki, et al. also recognized the affinity between ceramics and bone tissue but noted that ceramic prostheses are weak on impact resistance. Suzuki et al. therefore proposed metal prosthesis plasma sprayed with a bonding agent which is in turn covered with a porous cement coating which will allow the ingrowth of bone spincules into the pores. This combination, it was said, would provide both the mechanical strength of metals and the bio-compatibility of ceranics.
The Suzuki patent did not address the issue of friction or wear of orthopedic implant bearing surfaces but confined itself to the single issue of the biocompatibility of metal prostheses. Furthermore, Suzuki et al. did not address the issue of dimensional changes that occur when applying a coating or the effect of these dimensional changes in the tightness of fit between the surfaces of an articulating joint prosthesis.
In addition, the application of ceramic coating to metal substrates often results in non-uniform, poorly adhering coatings which tend to crack due to the differences in elastic modulus or thermal expansion between the ceramic and underlying metal substrate. Further,or; such coatings tend to be relatively thick (50-300 microns) and since the bond between the metal and the ceramic coating is often weak, there is the risk of galling or separation of ceramic coatings.
Previous attempts have been made to produce zirconium oxide coatings on zirconium pans for the purpose of increasing their abrasion resistance. One such process is disclosed in U.S. Pat. No. 3,615,885 to Watson which discloses a procedure for developing thick (up to 0.23 mm) oxide layers on Zircaloy 2 and Zircaloy 4. However, this procedure results in significant dimensional changes especially for parts having a thickness below about 5 mm, and the oxide film produced does not exhibit especially high abrasion resistance.
U.S. Pat. No. 2,987,352 to Watson discloses a method of producing a blue-black oxide coating on zirconium alloy parts for the purpose of increasing their abrasion resistance. Both U.S. Pat. No. 2,987,352 and U.S. Pat. No. 3,615,885 produce a zirconium dioxide coating on zirconium alloy by means of air oxidation. U.S. Pat. No. 3,615,885 continues the air oxidation long enough to produce a beige coating of greater thickness than the blue-black coating of U.S. Pat. No. 2,987,352. This beige coating does not have not have the water resistance of the blue-black coating and is thus not applicable to many parts where there are two work faces in close proximity. The beige coating wears down more quickly than the blue-black oxide coating with the resulting formation of zirconium oxide particles and the loss of the integrity of the zirconium oxide surface. With the loss of the oxide surface the zirconium metal is then exposed to its environment and can lead to transport of zirconium ions away from the surface of the metal into the adjacent environment.
The blue-black oxide coatings have a thickness which is less than that of the beige coating although the hardness of the blue-black coating is higher than that of the beige coating. This harder blue-black oxide coating lends itself better to surfaces such as prosthetic devices. Although the blue-black coating is more abrasion resistant than the beige coating it is a relatively thin coating. It is therefore desirable to produce the blue-black coatings of increased abrasion resistance without producing the same type coatings of the prior art.
U.S. Pat. No. 5,037,438 to Davidson discloses a method of producing zirconium alloy prostheses with a zirconium oxide surface. U.S. Pat. No. 2,987,352 to Watson discloses a method of producing zirconium bearings with a zirconium oxide surface. The oxide coating produced is not always uniform in thickness and the non-uniformity reduces the integrity of the bonding between the zirconium alloy and the oxide layer and the integrity of the bonding with the oxide layer.
There exists a need for a method to produce oxide coatings form thickness on zirconium alloys. There exists a need for a metal alloy based orthopedic implant having low friction and highly wear resistant load bearing surfaces that can be implanted for the lifetime of the recipient. There also exists a need for a metal alloy based orthopedic implant that is not prone to corrosion by the action of the body fluids and is biocompatible and stable over the lifetime of the recipient.